<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>阶乘</title>
    <script>
    var num = prompt("请输入一个正整数",'');
			var a = 1;
			for (var i = num; i >0;i--) {
				a *= i;
			}
			alert(num + '的阶乘是' + a);
    </script>
</head>
<body>
    
</body>
</html>